The present invention relates to a color image forming apparatus. More specifically, the invention relates to a so-called tandem type color image forming apparatus provided with a plurality of monochromatic toner image carriers on an intermediate transfer belt or a recording medium delivery belt which is to be circulated, and serving to sequentially transfer toner images having different colors which are formed by a monochromatic toner image carrier to a recording medium delivered through the intermediate transfer belt or the recording medium delivery belt, thereby forming a color image. In particular, the invention relates to a driving system for rotating each monochromatic toner image carrier.
Japanese Patent Publication No. 61-156159A discloses a conventional tandem type color image forming apparatus.
As shown in FIG. 10A, the conventional color image forming apparatus comprises a recording medium delivery belt 1 to be circulated, three photosensitive members 2Y, 2M and 2C to be monochromatic toner image carriers (for three colors of yellow (Y), magenta (M) and cyan (C)) provided on the recording medium delivery belt 1, and a gear train 3 (see FIG. 10B) for rotating the monochromatic toner image carriers, and serves to sequentially transfer toner images carried by the three monochromatic toner image carriers 2Y, 2M and 2C to a recording medium P supplied through a paper feeding roller 4 and delivered through the recording medium delivery belt 1, thereby forming a color image on the recording medium P.
A driving gear 3a (Y, C, M) is provided on the end of each of the monochromatic toner image carriers 2Y, 2M and 2C.
Intermediate gears 3b and 3b are provided between the driving gears 3a (Y, C, M), and the driving gears 3a (Y, C, M) and the intermediate gears 3b and 3b are provided in a line to constitute a gear train 3.
A driving source gear 5a to be rotated by a motor 5 is engaged with the driving gear 3a (Y) positioned on the end of the gear train 3. Consequently, each of the monochromatic toner image carriers 2 (Y, M, C) is rotated through the gear train 3.
Thus, the driving gears 3a (Y, C, M) and the intermediate gears 3b and 3b are arranged in a line so that each of the monochromatic toner image carriers 2 (Y, C, M) can be rotated at a minimum gear number.
In the conventional apparatus, as shown in FIG. 10B, since the centers of the rotation of the driving gears 3a (Y, C, M) and the centers of the rotation of the intermediate gears 3b and 3b in the gear train 3 are arranged on a straight line SL, the following problem arises.
More specifically, the driving gear 3a and the intermediate gear 3b have manufacturing errors respectively and are swollen by a rise in a temperature in the apparatus. In some cases, therefore, the sizes of the driving gear 3a and the intermediate gear 3b are increased to be greater than desirable dimensions.
In such a situation, if the centers of the rotation of the driving gears 3a (Y, C, M) and the centers of the rotation of the intermediate gears 3b and 3b are arranged on the straight line SL, the gears 3a and 3b are pressed in the direction of the straight line SL and receive an unstable load when the sizes of the gears 3a and 3b are increased to be greater than the desirable dimensions due to the manufacturing error or the rise in the temperature.
Such an unstable load, particularly, an unstable load acting on the driving gear 3a unstably moves each of the monochromatic toner image carriers 2 (Y, C, M) in the direction of the straight line SL, causing an uneven rotation or a color shift. As a result, obtained image quality is deteriorated.
In the conventional apparatus, with respect to the drive gear 3a (M) for driving the image carrier 2M, a driving force F sent from the intermediate gear 3b on the upstream side and a driving reaction force R sent from the intermediate gear 3b on the downstream side act.
On the other hand, since developing rollers 6 (Y, C, M) abut on the image carrier 2, an abutment force Fd acts on the image carrier 2.
In this configuration, since the driving force F, the driving reaction force R and the abutment force Fd sent from the developing roller (abutment member) 6 are sent in one direction (downward in FIG. 10B), a resultant force thereof acts as a great radial force on the bearing portion of the image carrier.
For this reason, the conventional apparatus has a problem in that a driving torque is increased and the bearing portion is apt to be deteriorated by a wear so that the rotation blurring of the image carrier is apt to be generated and image quality is thereby deteriorated easily.
Moreover, since the image carrier gear is provided on only one end of the image carrier, the great radial force acts on one end side thereof. The radial force on one end side (therefore, the imbalance of the radial force on both ends of the image carrier) causes the skew of the image carrier. When the image carrier skews, image forming conditions in an axial direction are changed. Therefore, there is a problem in that the image quality is deteriorated (the image quality in a transverse direction seen in the direction of delivery of the recording medium P is apparently varied).
The problems arise in a color image forming apparatus having one image carrier and a monochrome image forming apparatus as well as a tandem type image forming apparatus in which a plurality of image carriers are provided, and remarkably arise in a tandem type image forming apparatus in which a plurality of image carriers are provided.
As in the conventional apparatus, in the case in which the gear train 3 for driving a plurality of monochromatic toner image carriers 2 (Y, C, M) is constituted in a line, a color formed and transferred by the image carrier positioned in the final stage of the power transmission path causes a color shift or a jitter (a stripe-shaped density unevenness generated by a fluctuation in the speed of the image carrier) most easily by the accumulation of the manufacturing errors of the gears 3a (Y, C, M) and 3b constituting the gear train 3.
Moreover, in the case in which the gear train 3 is constituted in a line as shown in FIG. 10B, a driving force F sent from the intermediate gear 3b provided on the upstream side and a driving reaction force R sent from the intermediate gear 3b provided on the downstream side act on the driving gear (for example, 3a(M)) positioned on the way of the power transmission path, while only the driving force F sent from the intermediate gear 3b provided on the upstream side acts on the driving gear 3a(C) positioned in the final stage of the power transmission path. Therefore, the driving gears 3a(Y) and 3a(M) positioned on the way of the power transmission path and the driving gear 3a(C) positioned in the final stage are deformed differently.
For this reason, a color formed and transferred by the image carrier positioned in the final stage of the power transmission path causes the color shift or the jitter most easily.
Under such a situation, in the conventional apparatus, the driving gear 3a(C) of the image carrier 2(C) for cyan to be a relatively remarkable color is provided in the final stage of the power transmission path in the gear train 3. Therefore, the color shift and the jitter of the cyan image are easily remarkable in such a state that the toner images having a plurality of colors are superposed on the recording medium P. As a result, there is a problem in that the image quality of the whole color image is deteriorated.
As shown in FIG. 11, Japanese Patent Publication No. 11-231754A discloses a color image forming apparatus of such a type (an intermediate transfer type) comprising an intermediate transfer belt 6, and a plurality of monochromatic toner image carriers 2 (Y, M, C, K (black)) provided on the intermediate transfer belt 6, and serving to sequentially transfer toner images having different colors which are carried by the monochromatic toner image carriers to the intermediate transfer belt 6, so that the toner images are superposed thereon to form a color image. The color image is collectively transferred from the intermediate transfer belt 6 to a recording medium P at a secondary transfer portion T2.
In the color image forming apparatus using an electrophotographic technique, generally, there is sometimes caused such a phenomenon that a toner slightly sticks to a portion to which the toner should not originally stick (a so-called fog phenomenon).
In such a situation, in a structure in which an image carrier 2K for a black toner image which is the most remarkable is provided as a monochromatic toner image carrier for finally transferring a toner image to the intermediate transfer belt 6 to be the intermediate transfer member as in the apparatus of an intermediate transfer member type shown in FIG. 11, for example, a black toner TK is also transferred onto the recording medium P continuously together with the yellow toner TY as shown in FIG. 12A when the fog phenomenon is generated so that the black toner TK sticks onto the yellow toner TY in a portion in which the yellow image is to be originally formed and is then transferred collectively to the recording medium P in the secondary transfer portion T2 as shown in FIG. 12B, for example. As a result, there is such a drawback that the black color is remarkable in a color image on the recording medium P.
It is the first object of the invention to provide a color image forming apparatus capable of decreasing the unstable load to reduce the unstable movement of each monochromatic toner image carrier and of reducing an uneven rotation and a color shift, thereby enhancing the image quality.
It is the second object of the invention to provide a color image forming apparatus capable of reducing a driving torque, causing a bearing portion to be deteriorated by a wear with difficulty, and enhancing image quality.
It is the third object of the invention to provide a color image forming apparatus capable of reducing a color shift or a jitter, thereby enhancing image quality.
It is the fourth object of the invention to provide a color image forming apparatus capable of causing a color shift or a jitter to be unremarkable, thereby enhancing image quality.
In order to achieve the above objects, according to the present invention, there is provided a color image forming apparatus, comprising:
a circulating belt member,
a plurality of monochrome toner image carriers, respectively opposing to the belt member to transfer each monochrome toner image onto the belt member or a recording medium held on the belt member;
a plurality of driving gears, each rotating one of the monochrome toner image carriers; and
a plurality of intermediate gears, each disposed between adjacent driving gears,
wherein a rotation center of each intermediate gear is placed at a position shifted from a line connecting rotation centers of the adjacent driving gears.
In this configuration, even if at least one of the sizes of the driving gear and the intermediate gear is increased to be greater than the desirable dimensions due to the manufacturing error or the swelling caused by the rise in the temperature in the apparatus, the driving gear is not directly pressed in the direction of the line connecting the rotation centers thereof.
Accordingly, an unstable load in the linear direction which acts on the driving gear is decreased so that the unstable movement of each monochromatic toner image carrier is also reduced.
As a result, according to the color image forming apparatus in accordance with the first aspect of the invention, the uneven rotation and color shift of the monochromatic toner image carrier can be reduced so that the image quality can be enhanced.
Preferably, each intermediate gear is movable in a direction perpendicular to the line connecting the rotation centers of the adjacent driving gears, when the intermediate gear is engaged with the adjacent driving gears.
In this configuration, even if at least one of the sizes of the driving gear and the intermediate gear is increased to be greater than the desirable dimension due to the manufacturing error or the swelling caused by the rise in the temperature in the apparatus, the intermediate gear is moved away in the direction orthogonal to the line connecting the rotation centers of the driving gears provided on both sides thereof in engagement with the driving gears.
Accordingly, the unstable load in the linear direction which acts on the driving gear is decreased still more and the unstable movement of each monochromatic toner image carrier is also reduced still more. As a result, the uneven rotation and color shift of each monochromatic toner image carrier can be reduced still more so that image quality can be enhanced still more.
Here, it is preferable that the apparatus further comprises a plurality of elastic members, each urging one of the intermediate gears in the direction perpendicular to the line connecting the rotation centers of the adjacent driving gears.
In this configuration, the engagement of the intermediate gear with the driving gear and the movement of the intermediate gear in the direction orthogonal to the line connecting the rotation centers of the driving gears can smoothly be carried out.
Alternatively, it is preferable that the apparatus further comprises a plurality of shafts, each inserted through a center hole formed in each intermediate gear for rotatably supporting the intermediate gear. Here, a clearance is formed between an outer face of the shaft and an inner face of the center hole.
In this configuration, the intermediate gear can be moved corresponding to the clearance without depending on the urging force of the elastic member in the direction orthogonal to the line connecting the rotation centers of the driving gears.
Preferably, the apparatus further comprises a common frame which supports the monochrome toner image carriers and the driving gears as a unit.
In this configuration, the monochromatic toner image carriers can be collectively attached to and removed from an apparatus body so that a maintenance property can be enhanced.
Furthermore, since a space between the rotation centers of the driving gears can be reduced, it is possible to reduce the size of the unit. As a result, the size of the whole apparatus can be reduced.
Here, it is preferable that the common frame supports the intermediate gears.
In this configuration, the state of reliable and smooth engagement of each driving gear and the intermediate gear can be maintained and the smooth and reliable rotation of each of the monochromatic toner image carriers can be ensured.
Preferably, a monochrome toner image carrier for transferring a least remarkable color toner image is driven by a drive gear placed at a final stage of a gear train including the drive gears and the intermediate gears.
Here, it is preferable that a monochrome toner image carrier for transferring a most remarkable color image is driven by a drive gear placed at an initial stage of a gear train including the drive gears and the intermediate gears.
Still here, it is preferable that the least remarkable color is yellow and the most remarkable color is black.
In the above configurations, even if a color shift or a jitter is occurred on a color formed and transferred by the monochromatic toner image carrier to be driven by the driving gear positioned in the final stage of the power transmission path, the shift or jitter of the image having the color becomes unremarkable in such a state that the toner images having a plurality of colors are formed on the recording medium. As a result, the resultant image quality of the whole color image can be enhanced.
Furthermore, if a fog phenomenon is occurred so that the yellow toner sticks onto the black toner in a portion in which a black image is to be originally formed, for example. Then, when the toner image is collectively transferred onto the recording medium in the secondary transfer portion, the yellow toner is covered with the black toner which is more remarkable (a toner which is more remarkable than the yellow toner and has a deeper color, and is not restricted to the black toner) over the recording medium even if the yellow toner is also transferred onto the recording medium continuously together with the black toner. As a result, the yellow color becomes unremarkable in a color image formed on the recording medium.
Therefore, the image quality of the whole color image can be enhanced still more.
Preferably, the color image forming apparatus further comprises at least one abutment member which abuts against each monochrome toner image carrier. Here, a resultant force obtained from at least one abutment force from the abutment member to one associated monochrome tone image carrier is oriented substantially opposite to a power transmission force from one drive gear for rotating the one monochrome toner image carrier to one subsequent intermediate gear.
Preferably, the resultant force obtained from the at least one abutment force is oriented substantially opposite to a resultant force obtained from the power transmission force and a reaction force acting to the one intermediate gear from one subsequent drive gear.
In the above configuration, In the above configurations, a power transmission force sent from the intermediate gear to the driving gear and a force acting on the image carrier by the abutment member are at least partially canceled with each other. Accordingly, a radial force acting as a resultant force on the bearing portion of the image carrier is more reduced than that of the conventional apparatus.
Therefore, a driving torque is reduced, a bearing portion is also deteriorated by a wear with difficulty and the rotation blurring of the image carrier is generated with difficulty so that image quality is deteriorated with difficulty.
Moreover, since the imbalance state of the radial force on both ends of the image carrier is relieved, the skew of the image carrier is also reduced. As a result, the image quality can be enhanced.
Preferably, a drive gear for rotating a monochrome toner image carrier, placed at a final stage of a gear train including the drive gears and the intermediate gears, circulates the belt member.
In this configuration, a driving force sent from the intermediate gear provided on the upstream side and a driving reaction force sent from the driven member side on the downstream side act on the driving gear for the, monochromatic toner image carrier in the final stage.
Consequently, a degree of deforming the driving gear positioned in the final stage of the power transmission path can be prevented from being greatly different from a degree of deforming the driving gear positioned on the way of the power transmission path.
Accordingly, the rotating state of the monochromatic toner image carrier positioned in the final stage of the power transmission path is more stabilized than that in the conventional apparatus, and a color shift or a jitter is generated with difficulty on a toner image formed and transferred by the image carrier. As a result, the image quality of a whole color image can be enhanced.
According to the present invention, there is also provided a color image forming apparatus, comprising:
a circulating belt member;
a plurality of monochrome toner image carriers, respectively opposing to the belt member to transfer each monochrome toner image onto the belt member or a recording medium held on the belt member;
a plurality of driving gears, each rotating one of the monochrome toner image carriers; and
a plurality of intermediate gears, each disposed between adjacent driving gears for transmitting a driving force from a previous driving gear to a subsequent driving gear,
wherein a monochrome toner image carrier for transferring a least remarkable color toner image is placed at a final stage of the monochrome toner image carriers with regard to a circulating direction of the belt member.
According to the present invention, there is also provided a color image forming apparatus, comprising:
a circulating belt member,
a plurality of monochrome toner image carriers, respectively opposing to the belt member to transfer each monochrome toner image onto the belt member or a recording medium held on the belt member;
a plurality of driving gears, each rotating one of the monochrome toner image carriers;
a plurality of intermediate gears, each disposed between adjacent driving gears for transmitting a driving force from a previous driving gear to a subsequent driving gear;
at least one abutment member which abuts against each monochrome toner image carrier,
wherein a resultant force obtained from at least one abutment force from the abutment member to one associated monochrome tone image carrier is oriented substantially opposite to a power transmission force from one drive gear for rotating the one monochrome toner image carrier to one subsequent intermediate gear.
According to the present invention, there is also provided a color image forming apparatus, comprising:
a circulating belt member;
a plurality of monochrome toner image carriers, respectively opposing to the belt member to transfer each monochrome toner image onto the belt member or a recording medium held on the belt member;
a plurality of driving gears, each rotating one of the monochrome toner image carriers; and
a plurality of intermediate gears, each disposed between adjacent driving gears for transmitting a driving force from a previous driving gear to a subsequent driving gear,
wherein a drive gear for rotating a monochrome toner image carrier, placed at a final stage of a gear train including the drive gears and the intermediate gears, drives another driven member.